a) Field of the Invention
The present invention relates to vectors and methods for the introduction of exogenous genetic material into avian cells and the expression of the exogenous genetic material in the cells. The invention also relates to transgenic avian species, including chickens, and to avian eggs which contain exogenous protein.
b) Description of Related Art
Numerous natural and synthetic proteins are used in diagnostic and therapeutic applications; many others are in development or in clinical trials. Current methods of protein production include isolation from natural sources and recombinant production in bacterial and mammalian cells. Because of the complexity and high cost of these methods of protein production, however, efforts are underway to develop alternatives. For example, methods for producing exogenous proteins in the milk of pigs, sheep, goats, and cows have been reported. These approaches suffer from several limitations, including long generation times between founder and production transgenic herds, extensive husbandry and veterinary costs, and variable levels of expression because of position effects at the site of the transgene insertion in the genome. Proteins are also being produced using milling and malting processes from barley and rye. However, plant post-translational modifications differ from vertebrate post-translational modifications, which often has a critical effect on the function of the exogenous proteins.
Like tissue culture and mammary gland bioreactors, the avian oviduct can also potentially serve as a bioreactor. Successful methods of modifying avian genetic material such that high levels of exogenous proteins are secreted in and packaged into eggs would allow inexpensive production of large amounts of protein. Several advantages of such an approach would be: a) short generation times (24 weeks) and rapid establishment of transgenic flocks via artificial insemination; b) readily scaled production by increasing flock sizes to meet production needs; c) post-translational modification of expressed proteins; 4) automated feeding and egg collection; d) naturally sterile egg-whites; and e) reduced processing costs due to the high concentration of protein in the egg white.
The avian reproductive system, including that of the chicken, is well described. The egg white of the hen consists of several layers which are secreted upon the yolk during its passage through the oviduct. The production of an egg begins with formation of the large yolk in the ovary of the hen. The unfertilized oocyte is positioned on top of the yolk sac. Upon ovulation or release of the yolk from the ovary, the oocyte passes into the infundibulum of the oviduct where it is fertilized if sperm are present. It then moves into the magnum of the oviduct which is lined with tubular gland cells. These cells secrete the egg-white proteins, including ovalbumin, lysozyme, ovomucoid, conalbumin, and ovomucin, into the lumen of the magnum where they are deposited onto the avian embryo and yolk.
The ovalbumin gene encodes a 45 kD protein that is specifically expressed in the tubular gland cells of the magnum of the oviduct (Beato, Cell 56:335-344 (1989)). Ovalbumin is the most abundant egg white protein, comprising over 50 percent of the total protein produced by the tubular gland cells, or about 4 grams of protein per large Grade A egg (Gilbert, “Egg albumen and its formation” in Physiology and Biochemistry of the Domestic Fowl, Bell and Freeman, eds., Academic Press, London, New York, pp. 1291-1329). The ovalbumin gene and over 20 kb of each flanking region have been cloned and analyzed (Lai et al., Proc. Natl. Acad. Sci. USA 75:2205-2209 (1978); Gannon et al., Nature 278:428-424 (1979); Roop et al., Cell 19:63-68 (1980); and Royal et al., Nature 279:125-132 (1975)).
Much attention has been paid to the regulation of the ovalbumin gene. The gene responds to steroid hormones such as estrogen, glucocorticoids, and progesterone, which induce the accumulation of about 70,000 ovalbumin mRNA transcripts per tubular gland cell in immature chicks and 100,000 ovalbumin mRNA transcripts per tubular gland cell in the mature laying hen (Palmiter, J. Biol. Chem. 248:8260-8270 (1973); Palmiter, Cell 4:189-197 (1975)). DNAse hypersensitivity analysis and promoter-reporter gene assays in transfected tubular gland cells defined a 7.4 kb region as containing sequences required for ovalbumin gene expression. This 5′ flanking region contains four DNAse I-hypersensitive sites centered at −0.25, −0.8, −3.2, and −6.0 kb from the transcription start site. These sites are called HS-I, -II, -III, and —IV, respectively. These regions reflect alterations in the chromatin structure and are specifically correlated with ovalbumin gene expression in oviduct cells (Kaye et al., EMBO 3:1137-1144 (1984)). Hypersensitivity of HS-II and -III are estrogen-induced, supporting a role for these regions in hormone-induction of ovalbumin gene expression.
HS-I and HS-II are both required for steroid induction of ovalbumin gene transcription, and a 1.4 kb portion of the 5′ region that includes these elements is sufficient to drive steroid-dependent ovalbumin expression in explanted tubular gland cells (Sanders and McKnight, Biochemistry 27: 6550-6557 (1988)). HS-I is termed the negative-response element (“NRE”) because it contains several negative regulatory elements which repress ovalbumin expression in the absence of hormone (Haekers et al., Mol. Endo. 9:1113-1126 (1995)). Protein factors bind these elements, including some factors only found in oviduct nuclei suggesting a role in tissue-specific expression. HS-II is termed the steroid-dependent response element (“SDRE”) because it is required to promote steroid induction of transcription. It binds a protein or protein complex known as Chirp-I. Chirp-I is induced by estrogen and turns over rapidly in the presence of cyclohexamide (Dean et al., Mol. Cell. Biol. 16:2015-2024 (1996)). Experiments using an explanted tubular gland cell culture system defined an additional set of factors that bind SDRE in a steroid-dependent manner, including a NFκB-like factor (Nordstrom et al., J. Biol. Chem. 268:13193-13202 (1993); Schweers and Sanders, J. Biol. Chem. 266: 10490-10497 (1991)).
Less is known about the function of HS-III and -IV. HS-III contains a functional estrogen response element, and confers estrogen inducibility to either the ovalbumin proximal promoter or a heterologous promoter when co-transfected into HeLa cells with an estrogen receptor cDNA. These data imply that HS-III may play a functional role in the overall regulation of the ovalbumin gene. Little is known about the function of HS-IV, except that it does not contain a functional estrogen-response element (Kato et al., Cell 68: 731-742 (1992)).
There has been much interest in modifying eukaryotic genomes by introducing foreign genetic material and/or by disrupting specific genes. Certain eukaryotic cells may prove to be superior hosts for the production of exogenous eukaryotic proteins. The introduction of genes encoding certain proteins also allows for the creation of new phenotypes which could have increased economic value. In addition, some genetically-caused disease states may be cured by the introduction of a foreign gene that allows the genetically defective cells to express the protein that it can otherwise not produce. Finally, modification of animal genomes by insertion or removal of genetic material permits basic studies of gene function, and ultimately may permit the introduction of genes that could be used to cure disease states, or result in improved animal phenotypes.
Transgenesis has been accomplished in mammals by several different methods. First, in mammals including the mouse, pig, goat, sheep and cow, a transgene is microinjected into the pronucleus of a fertilized egg, which is then placed in the uterus of a foster mother where it gives rise to a founder animal carrying the transgene in its germline. The transgene is engineered to carry a promoter with specific regulatory sequences directing the expression of the foreign protein to a particular cell type. Since the transgene inserts randomly into the genome, position effects at the site of the transgene's insertion into the genome may variably cause decreased levels of transgene expression. This approach also requires characterization of the promoter such that sequences necessary to direct expression of the transgene in the desired cell type are defined and included in the transgene vector (Hogan et al. Manipulating the Mouse Embryo, Cold Spring Harbor Laboratory, NY (1988)).
A second method for effecting animal transgenesis is targeted gene disruption, in which a targeting vector bearing sequences of the target gene flanking a selectable marker gene is introduced into embryonic stem (“ES”) cells. Via homologous recombination, the targeting vector replaces the target gene sequences at the chromosomal locus or inserts into interior sequences preventing expression of the target gene product. Clones of ES cells bearing the appropriately disrupted gene are selected and then injected into early stage blastocysts generating chimeric founder animals, some of which bear the transgene in the germ line. In the case where the transgene deletes the target locus, it replaces the target locus with foreign DNA borne in the transgene vector, which consists of DNA encoding a selectable marker useful for detecting transfected ES cells in culture and may additionally contain DNA sequences encoding a foreign protein which is then inserted in place of the deleted gene such that the target gene promoter drives expression of the foreign gene (U.S. Pat. Nos. 5,464,764 and 5,487,992 (M. P. Capecchi and K. R. Thomas)). This approach suffers from the limitation that ES cells are unavailable in many mammals, including goats, cows, sheep and pigs. Furthermore, this method is not useful when the deleted gene is required for survival or proper development of the organism or cell type.
Recent developments in avian transgenesis have allowed the modification of avian genomes. Germ-line transgenic chickens may be produced by injecting replication-defective retrovirus into the subgerminal cavity of chick blastoderms in freshly laid eggs (U.S. Pat. No. 5,162,215; Bosselman et al., Science 243:533-534 (1989); Thoraval et al., Transgenic Research 4:369-36 (1995)). The retroviral nucleic acid carrying a foreign gene randomly inserts into a chromosome of the embryonic cells, generating transgenic animals, some of which bear the transgene in their germ line. Unfortunately, retroviral vectors cannot harbor large pieces of DNA, limiting the size and number of foreign genes and foreign regulatory sequences that may be introduced using this method. In addition, this method does not allow targeted introduction or disruption of a gene by homologous recombination. Use of insulator elements inserted at the 5′ or 3′ region of the fused gene construct to overcome position effects at the site of insertion has been described (Chim et al., Cell 74:504-514 (1993)).
In another approach, a transgene has been microinjected into the germinal disc of a fertilized egg to produce a stable transgenic founder bird that passes the gene to the F1 generation (Love et al. Bio/Technology 12:60-63 (1994)). This method has several disadvantages, however. Hens must be sacrificed in order to collect the fertilized egg, the fraction of transgenic founders is low, and injected eggs require labor intensive in vitro culture in surrogate shells.
In another approach, blastodermal cells containing presumptive primordial germ cells (“PGCs”) are excised from donor eggs, transfected with a transgene and introduced into the subgerminal cavity of recipient embryos. The transfected donor cells are incorporated into the recipient embryos generating transgenic embryos, some of which are expected to bear the transgene in the germ line. The transgene inserts in random chromosomal sites by nonhomologous recombination. This approach requires characterization of the promoter such that sequences necessary to direct expression of the transgene in the desired cell type are defined and included in the transgene vector. However, no transgenic founder birds have yet been generated by this method.
A targeting vector containing flanking DNA sequences of the vitellogenin gene has been used to delete part of the resident gene in chicken blastodermal cells in culture. However, it has not been demonstrated that these cells can contribute to the germ line and thus produce a transgenic embryo. In addition, this method is not useful when the deleted gene is required for survival or proper development of the organism or cell type.
Thus, it can be seen that there is a need for a method of introducing foreign DNA which is operably linked to a magnum-active promoter into the avian genome. There is also a need for a method of introducing foreign DNA into nonessential portions of a target gene of the avian genome such that the target gene's regulatory sequences drive expression of the foreign DNA, preferably without disrupting the function of the target gene. The ability to effect expression of the integrated transgene selectively within the avian oviduct is also desirable. Furthermore, there exists a need to create germ-line modified transgeneic birds which express exogenous genes in their oviducts and secrete the expressed exogenous proteins into their eggs.